Do not update result of *-config-show with empty server attributes

If a server attribute such as DNSSec Key master is unset, None is passed as
the attribute value into the upper API layers and displayed in the output of
`dnsconfig-show` et al. We should not show this and leave the attribute empty
instead.

https://fedorahosted.org/freeipa/ticket/5960

Reviewed-By: Pavel Vomacka <pvomacka@redhat.com>
Reviewed-By: Martin Basti <mbasti@redhat.com>

ipaserver/plugins/serverroles.py

@@ -134,9 +134,11 @@ class serverroles(Backend):
         except NotImplementedError:
             return result
 
-        result.update(
-            {name: attr.get(self.api) for name, attr in
-             assoc_attributes.items()})
+        for name, attr in assoc_attributes.items():
+            attr_value = attr.get(self.api)
+
+            if attr_value is not None:
+                result.update({name: attr_value})
 
         return result